Wood based panels such as plywood, flake board or fiber board have generally a balanced construction. This means that in thickness and in composition, both halves of the panel from their respective outer surfaces to the center plane are identical. For example, in a particle board or flake board panel, each half will have about the same amount and distribution of fine and course material, as well as the same distribution of density, and in a plywood panel the grain direction of the outer veneer layers are usually parallel to each other and these veneer layers are generally of the same thickness and species.
If however the product is not symmetrical, such as a plywood panel having face veneers of different thicknesses or species, or the symmetry is lost during the process, such as in a flake board panel which is cured under different temperatures on opposing faces, or the symmetry is lost by additions such as an overlay, applied to only one side of a panel, then such a panel will often acquire a curvature which may impede its future use. Especially "Plank-Ply" panels made as described in my U.S. Pat. No. 3,878,017 have the tendency to bow across the direction of the face grain, resulting usually in concave faces. These panels have faces of lumber planks or veneer strips which may vary in thickness and are produced in matching pairs, whereby a thick face plank matches a thin face plank in the mating panel during the laminating process.
Some wood based panels can be shaped after their manufacture by bending a panel under controlled conditions beyond its elastic limit. This may for example be done with a number of rollers such as are used when bending sheet metal or iron plates. However, using this method, the points of contact between the panel and the individual rollers should be close together, since otherwise, the tension side of the panel will develop large cracks. This limits the diameter of the rollers and permits only to shape relatively thin panels. Another disadvantage of this method is that panels having thickness variations such as "Plank-Ply" cannot be shaped successfully by such rollers unless the roller system has complicated adjustment means to accommodate the varying thickness of the panels.
Another method of flattening wood based panels is to balance one factor, such as an overlay on only one side of the panel, with another factor, such as a higher moisture content in the veneer or particle mat of the other side of the panel prior to pressing. This will sometimes produce satisfactory results, but is very sensitive to many other conditions of the process and therefore unreliable.
Most panels, including lumber plank faced panels, where the lumber planks may differ from panel to panel in thickness, density, grain direction, species and moisture content, require a more reliable method of flattening. Generally, a slightly convex faced panel can be applied to a wall with little difficulties and may sometimes be even more desirable than an absolutely flat panel. A panel with concave faces will tend to pull away from the wall along the edges and cause considerably more trouble. The term flattening used in this description shall include the action of changing a concave faced panel of various degrees into a less concave or flat faced panel, and shall also include changing such a panel into a convex faced panel.